Camera sensors are used in mobile devices, wireless devices or camera devices. All the pixels in the camera sensors, such as CMOS (complimentary metal oxide semiconductor) or CCD (charge coupled device) sensors, are not performing similarly. Some of the pixels are jammed to one value. Some of pixels are responding faster or slower than the others, etc. Those pixels that are operating differently than the other neighbor pixels are classified as pixel defects. In the image sensor there can be singlet defects, couplet defects or defect clusters. A singlet defect means that there is no other defect pixel, e.g., in the same colored 3×3 pixel neighbors. A couplet defect means that there are two colored defect pixels of the same color connected to each other and the defect clusters mean that there are more than two defects connected to each other.
The problem is how the defects can be removed from the images without blurring the image and also with a minimal amount of processing power and memory required for the removal operation. The problem is especially related to the singlet or couplet defect removal, because the defect clusters are usually too difficult to remove without losing some real details from the image.
There are many different methods for removing those defects. For example, the defects can be removed from a resultant image, but this is not a good place for this kind of operation because the defect values are already spread to the other pixels after, e.g., CFA (color filter array) interpolation. Some manufacturers are removing the defects in the camera sensor module (e.g., using CMOS sensor) or in a separate processing chip simultaneously with other image processing. Typically, there are two different methods used. The first one is a median based filter and it can sometimes remove also defect clusters (3×3) (e.g., by STMICROELECTRONICS using a sensor) and sometimes only singlet defects (3×1, 1×3) (e.g., by TEXAS INSTRUMENT using hardware). The other method is based on minimum and/or maximum filter and it can remove only the singlet pixel defects (e.g., by TEXAS INSTRUMENT using software). Sometimes there is also a set of different filters used to remove those defects (e.g. used by STMICROELECTRONICS in hardware accelerator) Quite often in DSCs (digital still cameras), that use most often CCD sensors but sometimes also CMOS sensors, the LUT (look-up table) based method for defect removal is used. This LUT based method means that the defect pixel are searched, e.g., during a camera calibration and then their coordinates are stored to the LUT. This LUT is used for each image and the known defects recorded in the LUT are filtered away. This method provides better quality because it does not smooth the other pixels than the defects, but it requires separate camera calibration.